1. Field of the Invention
The present invention relates to a fuel cell stack including a plurality of fuel cells stacked together and end plates provided at opposite ends of the fuel cells in the stacking direction. Each of the fuel cells includes a membrane electrode assembly and a separator stacked together. The membrane electrode assembly includes a pair of electrodes and an electrolyte membrane interposed between the electrodes.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs an electrolyte membrane (electrolyte) comprising a polymer ion exchange membrane. The electrolyte membrane is interposed between an anode and a cathode to form a membrane electrode assembly (MEA). The membrane electrode assembly is sandwiched between separators to form a unit of power generation cell (unit cell) for generating electricity. In use, generally, a predetermined number of such unit cells are stacked together to form a fuel cell stack.
In order to obtain the desired power generation performance and achieve the sealing function, a suitable tightening load needs to be applied to the fuel cell stack in the stacking direction.
In this regard, for example, in a fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2000-048850, as shown in FIG. 7, predetermined numbers of separators 1 and electrode units 2 are stacked together to form a stack body, and pressure plates 3a, 3b are provided at opposite ends of the stack body. Holding members 4 each having an L-shape in cross section are provided at respective corners of the pressure plates 3a, 3b. The holding members 4 are tightened to the pressure plates 3a, 3b using screws 5.
That is, a tightening load in the stacking direction is applied to the separators 1 and the electrode units 2 between the pressure plates 3a, 3b through the four holding members 4.
However, in Japanese Laid-Open Patent Publication No. 2000-048850, the holding members 4 and the pressure plates 3a, 3b are tightened together simply by the screws 5 inserted through the holding members 4 and screwed into screw holes formed on sides of the pressure plates 3a, 3b. 
In the structure, when a load is applied to the fuel cell from the outside, the outer shape of the stack body is deformed easily, and the desired tightening load in the stacking direction cannot be maintained. As a consequence, the tightening load applied to the stack body may become insufficient. Further, due to deformation in the outer shape of the stack body, the separators 1 and the electrode units 2 may move undesirably.